1. Field of the Invention
This invention relates to an installation and a process for the generation of electrical energy, whereby fuel cells perform the conversion of energy chemically bonded in a fuel into electrical energy.
2. Background Information
Fuel cells have been part of the prior art for many years. There are a number of different types, which are operated at different pressures, temperatures and with different electrolytes. Examples include alkaline fuel cells (AFC=Aklaline Fuel Cell), phosphoric acid fuel cells (PAFC=Phosphoric Acid Fuel Cell), molten carbonate fuel cells (MCFC=Molten Carbonate Fuel Cell), solid oxide fuel cells (SOFC=Solid Oxide Fuel Cell), or solid polymer electrolyte fuel cells (SPFC=Solid Polymer Electrolyte Fuel Cell). A fuel cell always has an anode chamber and a cathode chamber, between which an electric current flows through an electrolyte. The anode chamber generally contains hydrogen gas or another gas rich in H.sub.2 as the fuel, and the cathode chamber contains a gas containing O.sub.2 (in particular air) as the oxidizing agent.
An oxidation process then takes place in the fuel cell at a temperature level which is relatively low compared to thermal combustion, for which reason we also speak of the "cold combustion" of the fuel. The efficiency of the fuel cell can generally be increased by increasing the operating pressure. Since its mechanical structure is very sensitive, precautions must be taken so that the pressure of the H.sub.2 -rich anode gas and the pressure of the cathode gas containing O.sub.2 are approximately equal to avoid mechanical damage. There must also be a control system to cool the fuel cell, so that the operating temperature always remains at the required level, independent of current fluctuations. An additional important point which has an effect on the operational safety of a fuel cell system is the maintenance of a sufficient degree of purity of the anode gas. For example, several types of fuel cells are sensitive to CO (e.g. PAFC), while others, such as MCFC or SOFC, are not.
To increase the overall efficiency of the electric current generation by means of fuel cells, and to achieve competitiveness with conventional processes for the generation of electric energy, the generation of the H.sub.2 -rich gas has heretofore been directly connected to the current generation, since in this manner the energy and fluid flows which occur in the fuel cell process can be utilized in the context of the conversion of a hydrocarbon into an H.sub.2 -rich gas. That would result in a close integration of the two subsystems, as will be explained below in greater detail on the basis of the schematic diagram in FIG. 1.
The fuel cell system designated B can consist of a single fuel cell, but also of several fuel cells connected together. (In the remainder of this description, the term "fuel cell" is also understood to include the possibility of several fuel cells.) This fuel cell B has two input gas currents, namely one H.sub.2 -rich anode gas current 2 and a cathode gas current 3 containing 02, which consists, for example, of compressed air. The compressed air, for example, can be supplied by an electrically operated compressor. The fluid currents 2 and 3 are held at the same pressure level by corresponding control devices, to prevent mechanical damage to the fuel cell. Exhaust gases are formed as a result of the chemical/physical processes taking place in the fuel cell. Since the H.sub.2 content of the H.sub.2 -rich gas 2 cannot be completely consumed, the anode exhaust gas current 4 discharged from the anode chamber still contains a residual amount of H.sub.2. Depending on the method of operation and the type of fuel cell, the remaining concentration is in the range of approximately 5-30% of the initial amount. The actual value is a function of the gas composition and the fuel consumption in the cell. The cathode exhaust gas current 6 being discharged from the cathode chamber also still contains a portion of the original O.sub.2 content of the cathode gas current 3 (frequently approximately one-half). Since during the "cold combustion" of the hydrogen in the fuel cell, a corresponding amount of water is formed, the water can be separated, e.g. by condensation of the cathode exhaust gas (in many fuel cells of the anode exhaust gas, too), in the form of high-purity water. In FIG. 1, the water current recovered is designated 5. Finally, a (fluid current 7 exits the fuel cell B, which is intended to symbolize the removal of heat, i.e. it represents the cooling system of the fuel cell B. Such a cooling system can be configured as an open or as a closed cooling system, in which the heat to be discharged is transferred to another medium. Systems of the prior art generally employ open cooling systems, also using them to generate steam. The cooling water to be used must be very carefully purified (better than standard demineralized boiler water).
Not only is that very expensive, but it frequently does not even achieve the desirable long-term operation of the cooling system, on account of the residual concentration of minerals always left in the water. By means of the fluid currents 4 to 7, the fuel cell system B is integrated into the generation of the H.sub.2 -rich anode gas, which takes place in the H.sub.2 -unit A. The H.sub.2 -unit A works mostly as a steam reformer installation. The raw material introduced into the reformer installation is a current of gaseous hydrocarbons 1 which is saturated with steam. At least some of the water 5 recovered from the waste gas of the fuel cell A can be used for that purpose. The heat which is given off during the cooling of the fuel cell B can be used to convert the water into the steam phase, and to superheat the steam. The H.sub.2 content (and the other combustible components such as CO and hydrocarbons) in the anode exhaust gas current 4 and the O.sub.2 content of the cathode exhaust gas current 6 are frequently used for combustion, to at least partly supply the heat requirement of an indirectly heated reformer in the H.sub.2 unit A, since the steam reforming process is strongly endothermal. Since the operating pressure of the fuel cell B is normally relatively high (approximately 2-10 bar), the steam reforming and frequently also the combustion for the indirect heating of the reformer are performed at correspondingly high pressures. On account of the refractory materials required, the costs for the fabrication of the steam reforming system are particularly high, and there are also increased safety problems.
The diagram in FIG. 1 is very rough and does not show any details. For example, it does not show that the product gas generated in the H.sub.2 -unit A, before it is introduced into the fuel cell B, is cooled and has generally been subjected to a prior CO/H.sub.2 shift treatment. The heat which is thereby given off is also used to heat the input fluid currents of the steam reforming process.
FIG. 1 shows the high degree of interconnection between the H.sub.2 unit A and the fuel cell system B. It shows that operating fluctuations of the one unit have direct effects on the other unit. While the electrochemical process in the fuel cell B can be influenced very quickly (practically instantly), the subsystem for the generation of an H.sub.2 -rich gas (H.sub.2 unit A) reacts to corresponding interventions very slowly (on the order of several minutes). For this reason, the startup phase and adjustments to different loads on the electricity discharge side present major problems, from the point of view of control and regulation. In spite of a great deal of effort and expense, the prior art has not been able to solve these problems, or to achieve satisfactory values for the duration of normal operation. On the 80 or so systems which have been constructed worldwide, the duration of problem-free operation is only several thousand hours, or even significantly less than that. Only very small systems have been able to operate for up to 20,000 hours. But the market requires a minimum operating time of 100,000 h and more.
The generic GB-A-21 82 195 discloses a process for electricity generation by means of fuel cells, in which, in comparison to the prior art illustrated in FIG. 1, the subsystems for the generation of the H.sub.2 -rich gas and the fuel cell are no longer so closely interwoven with one another, whereby the combustion exhaust gas generated is not used to heat an indirectly fired steam reformer unit, but is discharged to a gas turbine. This gas turbine drives a compressor which supplies the compressed air for the operation of the fuel cell, and if necessary, also for the performance of the catalytic combustion.
An additional important task for the pneumatic compressor is the supply of the steam reformer unit with combustion compressed air. In this process, a special reformer is used which contains a primary reformer stage and a secondary reformer stage. Between the two stages, a partial combustion takes place for heating in the product gas already produced, the oxygen for which must be supplied by the compressed air compressor.
With fuel cell systems of the prior art employing indirectly heated steam reformer units, the combustion for the indirect heating also frequently takes place under elevated pressure, so that the combustion air must be supplied in the form of compressed air.
An additional characteristic of the process disclosed in GB-A-21 82 195 is that the compressed air required for the combustion in the steam reformer--in a variant of the process and following saturation with steam--is preheated by the hot flue gases generated by the catalytic combustion of the exhaust gases of the fuel cell. The recovery of water from the cooled flue gases from the catalytic combustion for use as the raw material for the steam reforming is described only as a possible variant of the process. In summary, therefore, we find that between the system for the generation of a H.sub.2 -rich gas and the fuel cell system, in this process there are three additional fluid currents, namely the H.sub.2 feed current to the fuel cell, the compressed air current to the steam reformer and the heat current.(if the latter is also included as a "fluid current" in the broader sense). Thus, as before, there is a strong interdependence between the subsystems indicated above, so that the existing problems of control and regulation have still not been solved. The construction of the steam reformer remains complex and expensive since, like the entire system, it must be designed for the operating pressure of the fuel cell.